The present invention relates to the field of Light Emitting Diode (LED) driving systems and LED driving methods for lighting LEDs.
More specifically, the present invention relates to driving LEDs in lighting applications. Currently, LED driver circuits are complex and often include feedback circuits to keep the current through an LED constant. For example, U.S. Pat. No. 6,836,157 issued to Rader et al., teaches a plurality of LEDs driven in parallel, in at least two modes. In a first mode, the LEDs are driven with a first voltage. In subsequent modes, the LEDs are driven with successively higher voltages. The forward voltage drop for each LED is monitored, and the driver switches from the first mode to successive modes based on the largest of the LED forward voltage drops. The current through each LED is controlled by a reference current through a first digitally controlled variable resistance circuit, and directing the LED current through a second digitally controlled variable resistance circuit having substantially a known ratio to the first variable resistance circuit and connected in series with the LED. A digital count is altered based on a comparison of the first and second currents, and the first and second variable resistance circuits are simultaneously altered based on the digital count.
However, in some lighting applications, multiple LED's are driven together to achieve the requisite luminosity. In these lighting applications the LED's are activated and light up simultaneously. For example, U.S. Pat. No. 6,756,893 issued to Fernandez teaches a plurality of light emitting diodes that are mounted on a base that surrounds a primary vehicle light source such as a headlight. A control circuit senses when the primary light source fails to provide the requisite light, and applies power to the light emitting diodes so that the vehicle can continue safely without loss of the function served by the primary light source. LEDs are also used in applications such as garden lights or outdoor lights typically used in residential applications to light walkways or to provide decorative illumination in yards or gardens, as taught in US Patent Application Publication No. 2007/0091598 by Chen which discloses low voltage garden lights incorporating LEDs.
Another such example of LED applications is U.S. Pat. No. 5,896,084 issued to Weiss et al., which teaches a tail light assembly for a motor where at least one of the rear lights, the brake light and the turn signal light is comprised of LEDs and has a control device for operating the LEDs at a constant current for a given voltage range.
In addition, U.S. Pat. No. 7,685,753 issued to Slowski teaches an illuminated, shallow weatherproof signage character that has individual three dimensional back-lighted/front/side and/or silhouette-lighting, with miniature, LED lamps concealed in the character.
Other refinements to LED driver circuits include addition of dimming capability, improvements in current control, and design simplification. US Patent Application Publication No. US 2010/0295478 A1 to Wei et al. teaches an LED driving circuit adapted to couple with a power supply and phase control dimmer to provide stable dimming performance. Also, US Patent Application Publication No. US 2011/0140622 A1 to Suzuki describes an LED driving circuit that uses a phase-controlled dimming circuit for constant current drive with a bleeder circuit included. U.S. Pat. No. 7,977,891 to Shiu et al. teaches an LED driving circuit comprising a transistor switch and a feedback controller. The invention uses voltage sensing feedback for current control. US Patent Application Publication No. US 2009/0315480 A1 to Yan et al. is a brightness-adjustable LED driving circuit with power factor correction. US Patent Application Publication No. US2010/0072898 A1 to Ohashi et al. teaches an LED driving circuit with serially connected LEDs using a switching device to control current flow, and another patent, U.S. Pat. No. 7,138,770 to Uang et al. is an LED driving circuit that is directly activated by an AC power supply. The LEDs are directly driven by placement within the bridge rectifier circuit.
The above mentioned patents and applications teach regulation of LED drive current of an LED string but do not teach reconfiguring the LED string for control of drive current.
Most LEDs are current devices and operate at specific current levels that are functions of the supply voltage. The current increases rather quickly as the voltage increases beyond the optimum level. Conventional LED driver circuits are essentially power supply units that regulate either the output voltage or the current. They do not make use of the fact that many LEDs are typically strung together to have a reasonable voltage drop across the string.
There is a need for a more energy efficient LED driving method for such lighting. The methods and systems of the present invention provide the foregoing and other advantages.